Polybenzazole fibers have high strength and high modulus which are vastly greater than those of known super fibers Dyneema and Kevlar (both being trademarks). Heretofore, the high strength and high modulus of polybenzazole fibers have prevented production of high quality crimped staple fibers. Continuous fibers are cut and the obtained non-crimped staple fibers have been carded. This method results in frequent occurrence of neps during carding, making high quality card web hard to obtain, and spun yarn and nonwoven fabric having good quality cannot be manufactured.
In addition, various high tenacity fibers including polybenzazole fiber have very high strength and high modulus as compared with conventional polyester, nylon and acrylic fibers, to the extent that problems occur when conventional processing steps for the above-mentioned known fibers are applied. For manufacturing staple fibers from continuous filaments, for example, synthetic fibers such as polyester and acrylic fibers are manufactured by feeding a continuous filaments tow in a crimper box to afford crimps, and cutting the filaments tow with a rotary cutter wherein cutting blades are radially installed on the periphery of a rotor drum with an approximately equal pitch with the blades facing the peripheral direction (Japanese Patent Publication No. 58568/1986). In this case, the angle of the cutting blades of the rotary cutter is generally set to 30.degree. for polyester fibers and 45.degree. for acrylic fibers. When the fibers having a filament tenacity of not less than 20 g/d are cut using the same rotary cutter blade, however, the damage to the cutter blade occurs in a short time, and the blade cannot stand a long time use.
It is therefore an object of the present invention to provide crimped polybenzazole staple fibers allowing production of a high quality card web with less neps, and a method for producing them.
Another object of the present invention is to provide a rotary cutter having a cutting blade permitting a long time use, which is used for cutting a continuous filaments tow of high tenacity fibers to give staple fibers.